LAS VEGAS - At least 58 people were killed and more than 400 injured when a heavily armed "lone wolf" gunman opened fire from a 32-floor hotel room on an open-air concert on the Las Vegas Strip in the deadliest mass shooting in US history.

The Islamic State group claimed the 64-year-old Nevada man behind the Sunday night massacre, Stephen Craig Paddock, was one of its "soldiers" but the FBI said it had found no such connection so far.

Police said Paddock, a retired accountant, killed himself before a SWAT team breached his room in the Mandalay Bay hotel overlooking the venue for the country music concert .

President Donald Trump denounced what he called "an act of pure evil" and said he would visit Las Vegas on Wednesday.

Las Vegas Sheriff Joseph Lombardo told reporters at least eight rifles had been recovered from the gunman's hotel room.

Paddock was believed to be a "lone wolf" assailant who acted alone, Lombardo said, declining to speculate as to what may have motivated the attack.

"I can't get into the mind of a psychopath at this point," he said.

Lombardo said Paddock had apparently used a hammer to smash the window of his hotel room before opening fire on the crowd below.

Police said around 22,000 people were attending the concert when bursts of automatic gunfire erupted shortly after 10:00 pm (0500 GMT).

Concert-goers screamed and fled in panic as a steady stream of bullets rained down from the hotel.

Lombardo said 58 people had been confirmed dead and 515 injured and he said the toll could rise. The authorities issued an appeal for blood donors.

The Islamic State said one of its "soldiers" who had "converted to Islam several months ago" was behind the shooting but provided no evidence to back up the claim.

The Federal Bureau of Investigation said it has found no such link so far.

"As this event unfolds we have determined to this point no connection with an international terrorist group," said Aaron Rouse, special agent in charge of the Las Vegas office of the FBI.

Paddock, who photographs showed as greying with a trimmed beard and moustache, was a former accountant and a licensed private pilot with no criminal record, according to ABC News.

Eric Paddock, the gunman's brother, said he was at a complete loss to understand what may have motivated the shooting .

"This is an asteroid falling out of the sky," Eric Paddock told CBS News.

He said his brother had "no religious affiliation, no political affiliation."

"He was my brother. He was a guy. He gambled," Eric Paddock said. He said his brother had no history of mental illness and was "not an avid gun guy at all."

Paddock resided in a new golf course development in the desert just outside Mesquite, Nevada, 80 miles northeast (130 kilometers) of Las Vegas .

Witnesses said Paddock opened fire with an initial long burst of gunfire, and then appeared to reload as he continued his spree.

"We heard (what) sounded like a glass breaking, so you looked around to see what's going on and then heard a pop, pop, pop," Monique Dekerf told CNN.

"You'd think for a moment okay we're fine, there's no more gunfire, then it starts again." Country music star Jason Aldean was on stage and near the end of his concert when the shooting began.

Aldean initially carried on playing when the first crackle of gunfire could be heard but then hurried off the stage.

Robert Hayes, a firefighter from Los Angeles who was in front of the stage, said he first thought the shots were some kind of equipment malfunction.

Once he realized what was going on, he joined the first responders, donning one of their vests.

"Honestly I probably pronounced 15-20 people" dead, he told Fox News. "It was pretty much like a war scene inside."

Emergency crews used anything to hand as makeshift stretchers, including tables and metal railings normally used to control the crowds, said Hayes.

Asked if he thought it was an inexperienced gunman, he responded: "With 30,000 people in the arena area, it was kind of like shooting goldfish ... He didn't have to be good."

The Las Vegas attack is the deadliest shooting in recent US history, exceeding the toll of 49 dead in an attack on the Pulse nightclub in Orlando, Florida in June 2016.

It was also the latest in a series of recent deadly attacks at concert venues.

Twenty-two people were killed while leaving an Ariana Grande concert in the northern English city of Manchester in May when a suicide bomber detonated a nail bomb in the foyer.

Ninety people were killed in November 2015 at the Bataclan venue in Paris during a concert by US band the Eagles of Death Metal. A shocked Aldean told his fans via Instagram that he and his band was safe.

"Tonight has been beyond horrific," the singer wrote.

"It hurts my heart that this would happen to anyone who was just coming out to enjoy what should have been a fun night."

Pope Francis said he was "deeply saddened" by the "senseless tragedy" while Britain's Prime Minister Theresa May called it an "appalling attack."